Allow the user to add Prestress contribution to stress tensor results

[baeumler]

Problem

Currently, when using a prestress tensor for FSI simulation, the prestress contributions are not added to the stress result in TPOST when writing out either the second Piola Kirchhoff, Cauchy, or vonMises stress tensor. This results in wrong stress output in the vtu files.

Solution

In VTKXML/TPOST: If the prestress tensor is assigned, add the prestress tensor S0 to the final stress outputs (Piola Kirchhoff, Cauchy and VonMises).
Maybe let user set a flag if they don't want the prestress added.

Additional context

No response

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